The present invention relates to document reproducing apparatus and more particularly to a document jam detector for a copier.
A known type of image copier uses dry toner powder which is selectively applied to the surface of a sheet of plain paper to form the image copy. The powder is fixed or fused to the paper by applying heat. The amount of thermal energy transferred to the applied toner powder is important. If an insufficient amount of thermal energy is used, the toner powder particles will not be completly fused to the paper surface. The resulting copy can be readily smeared or erased. If too much thermal energy is applied, the paper will scorch or, under abnormal or fault conditions, catch fire.
Two variables control the amount of thermal energy transferred within such a copier. The first variable is the temperature at the image fixing station or fuser section of the copier. The second variable is the length of time the paper remains in the fuser. While the fusing temperature might be reduced to reduce the risk of scorching or fire, many toners in use today do not fuse well at reduced temperatures. Moreover, the fuser temperature can be reduced ony if the paper remains in the fuser for a long period of time. An inherent disadvantage of a longer fixing time is that the copier must operate at a slower speed.
For the reasons given above, higher fuser temperatures are preferred. Reliable paper flow becomes more critical at such higher fuser temperatures.
While the possibility of jams in the paper flow path can be reduced through careful design, it cannot be completely eliminated. The possibility that documents may become jammed must still be considered in order to avoid having the paper catch fire within the fuser.